linux-kernel - Re: [PATCH] staging: pi433: add descriptions for mutex locks

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Message-ID: <20180412184608.GV3257@gavran.carpriv.carnet.hr>
Date:   Thu, 12 Apr 2018 20:46:08 +0200
From:   Valentin Vidic <Valentin.Vidic@...Net.hr>
To:     Marcus Wolf <marcus.wolf@...rthome-wolf.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Simon Sandström <simon@...anor.nu>,
        Marcus Wolf <linux@...f-Entwicklungen.de>,
        Luca Söthe <luca@...l.me>,
        Marcin Ciupak <marcin.s.ciupak@...il.com>,
        Michael Panzlaff <michael.panzlaff@....de>,
        Derek Robson <robsonde@...il.com>, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: pi433: add descriptions for mutex locks

On Sun, Apr 08, 2018 at 05:22:46PM +0200, Marcus Wolf wrote:
> Regarding your patch, I did not understand, why you did not remove
> the mutex_lock in pi433_write. Wasn't it the goal to remove it?

Is it possible for more than one userspace program to open the pi433
device and send messages?  In that case more than one pi433_write
could be running and it needs to hold a mutex_lock before calling
kfifo_in.

> Below find a proposal of pi433_write function, I wrote on base
> of my outdated (!), private repo. It is not compiled and not tested.
> Since there is no more handling in case of an error (as well in the
> propsal as in your patch), I removed the error handling completely.
> I only do a test to detect proplems while writing to the tx_fifo,
> but (like in your patch) do nothing for solving, just printing a line.
> If this unexpected situation will occur (most probably never),
> the tx_fifo will be (and stay) out of sync until driver gets unloaded.
> We have to decide, whether we can stay with that. Like written above,
> I thinkt the benefits are great, the chance of such kind of error
> very low.
> What do you think?

Yes, if there is only one writer and it checks the available size,
kfifo_in should not fail.  The only problem might be copy_from_user
but perhaps that is also quite unlikely.  A workaround for that could
be to copy the data into a temporary kernel buffer first and than
start kfifo writes using only kernel memory.

> It could be discussed, whether it is better to return EMSGSIZE or
> EAGAIN on the first check. On the one hand, there is a problem with
> the message size, on the other hand (if message isn't generally too
> big) after a while, there should be some more space available in
> fifo, so EAGAIN may be better choice.

EAGAIN does seem better unless the message is too big to ever fit
in the kfifo.

> 	if (retval != required ) {
> 		dev_dbg(device->dev, "write to fifo failed, reason unknown, non recoverable.");
> 		return -EAGAIN;
> 	}

Maybe this should be dev_warn or even dev_crit if the driver is not
usable anymore when this happens?  The error message should than also
be adjusted to EBADF or something similar.

-- 
Valentin